Electronic packaging technology has undergone repeated transformations and improvements during the past decade, driven by reliability, size, and price considerations. Most packaging methods involve the use of encapsulants to protect a semiconductor die and electrical interconnections such as bond wires from physical damage and ionic contamination prior to the final step of packaging the die.
One type of encapsulated electronic package is made by bonding one (or more) semiconductor die to a substrate such as a printed circuit board (PCB) or other wiring board with an adhesive, and covering the die with an encapsulant. The final packaging step includes encasing the die in a plastic molding to protect the die and fragile wire bonds from possible environmental damage. The encapsulants used to accomplish this are dispensed to form a glob over the die and the electrical interconnections, and are generally known to those skilled in the art as "glob top" encapsulants. The demand for miniaturized circuitry has led to the use of glob top as a preferred assembly method for many types of products, including microprocessor circuitry.
Glob-topping protects the semiconductor die and bonding wires that electrically couple the die to the circuit board or wiring board during manufacture. The encapsulant material and the process of applying the encapsulant are both referred to as glob top. The glob top encapsulant covers the die, portions of the metal circuit pattern on the surface of the die, the bond wires, and portion of the PCB surface. Conventional glob-topping processes rely upon the low viscosity of the encapsulant material at the forming temperature which is dispensed from a plastic syringe having a needle tip. In the glob top process, a dam is formed around the periphery of the die and bond wires to constrain the lateral flow of the encapsulant. The encapsulant material is then dispensed within the perimeter of the dam and gravity is relied upon to form the protective cover for the assembly. The syringe is used both to form the dam structure and dispense the additional glob top inside the dam. The material is dispensed from the syringe in the form of a viscous stream or column in a back and forth pattern until the areas that require protection have been completely covered with the glob top.
Because products are becoming more miniaturized to meet market demands even as integrated circuit die are growing larger with more outputs, the use of glob top material is becoming an attractive method of packaging because of its compact nature. For example, the surface area covered by the glob top material compared to standard encapsulated devices is much smaller. Furthermore, glob top material can induce significant stresses on the die that can lead to cracking.
Semiconductor die are not typically functionally and parametrically tested and burned in prior to assembly to a PCB. If a die that is assembled and glob-topped to a PCB later fails during testing it is difficult or impossible to remove and replace without damaging the PCB or bonding pads on the PCB. To overcome the problem of rework, a replacement (redundant) die is often assembled to the PCB while the nonfunctional die is left in place on the PCB but electrically disabled. Leaving the nonfunctional die in place, however, can defeat the size advantage offered by the glob top assembly as it requires additional space on the PCB and adding redundancy through additional die is rarely cost effective.
An assembly and process that reduces the problems associated with conventional glob top technology would be desirable.